When you change the request method by setting
CURLOPT_CUSTOMREQUEST(3) to something, you don't actually change how
libcurl behaves or acts in regards to the particular request method, it will
only change the actual string sent in the request.

Restore to the internal default by setting this to NULL.

This option can be used to specify the request:

HTTP

Instead of GET or HEAD when performing HTTP based requests. This is
particularly useful, for example, for performing an HTTP DELETE request.

Many people have wrongly used this option to replace the
entire request with their own, including multiple headers and POST
contents. While that might work in many cases, it will cause libcurl to
send invalid requests and it could possibly confuse the remote server
badly. Use CURLOPT_POST(3) and CURLOPT_POSTFIELDS(3) to
set POST data. Use CURLOPT_HTTPHEADER(3) to replace or extend the
set of headers sent by libcurl. Use CURLOPT_HTTP_VERSION(3) to
change HTTP version.

FTP

Instead of LIST and NLST when performing FTP directory listings.

IMAP

Instead of LIST when issuing IMAP based requests.

POP3

Instead of LIST and RETR when issuing POP3 based requests.

For example:

When you tell libcurl to use a custom request it will behave
like a LIST or RETR command was sent where it expects data to be
returned by the server. As such CURLOPT_NOBODY(3) should be used
when specifying commands such as DELE and NOOP for example.

SMTP

Instead of a HELP or VRFY when issuing SMTP based requests.

For example:

Normally a multiline response is returned which can be used,
in conjunction with CURLOPT_MAIL_RCPT(3), to specify an EXPN
request. If the CURLOPT_NOBODY(3) option is specified then the
request can be used to issue NOOP and RSET commands.

The application does not have to keep the string around after
setting this option.